1. Field of the Invention
The present invention relates to a rotor of a synchronous induction motor which is started using induction torque and synchronously operates using reluctance torque, a manufacturing method of the rotor, a die for the rotor, and a compressor employing the synchronous induction motor.
2. Description of the Related Art
FIG. 25 is a cross sectional view showing a rotor of a conventional synchronous induction motor, for example, a permanent magnetic synchronous motor disclosed in the Japanese Unexamined Patent Publication No. 57-180359. In the figure, a reference numeral 21 shows a rotor iron core, 22 shows a permanent magnet, and 24 shows a slot in which aluminum is filled by die-casting method, which form a cage secondary conductor with an end ring (not illustrated). Further, 23 shows a rotor cover made of nonmetallic material.
In the synchronous induction motor, aluminum is filled in the slot 24 to form a cage secondary conductor. Since the synchronous induction motor is started by a stator coil placed around the slot 24, there is no need to provide a special starting device. Further, after started, magnetic field which contributes to generation of torque can be formed by the permanent magnet 22. Since the synchronous induction motor does not need to be supplied electric current to excite, which enables to form the electric motor with high efficiency.
Four permanent magnets 22 are placed on an outer circumference of the rotor iron core, and the outer circumference of the permanent magnet 22 is covered with the nonmetallic rotor cover 23 to prevent the permanent magnet from being dispersed due to centrifugal force caused by the rotation of the rotor. Further, since the rotor cover 23 is made of nonmetallic material, eddy current is not generated on the surface of the rotor cover 23, which enables to obtain the permanent magnetic synchronous motor without eddy current loss.
As the conventional synchronous induction motor, the permanent magnet synchronous motor is formed as described above, and there are some problems as follows.
The permanent magnet 22 is placed on the outer circumference of the iron core 21 of the rotor, which creates problems such that when the rotor is driven at high speed, the motor vibrates or generates noise due to uneven processing of the circumference of the permanent magnet 21, or the motor does not have enough strength against the centrifugal force, and the motor does not last long. In order to reduce uneven processing, the processing precision must be increased, which results in an increment of the processing cost. Further, since the permanent magnets 22 are placed on the whole surface of the outer circumference of the iron core 21 of the rotor, all the permanent magnets 22 receive the centrifugal force. Accordingly, the rotor cover 23 must be covered the whole surface of the outer circumference of the rotor iron core, which requires an expensive rotor cover 23. If the rotor cover 23 is made thin to reduce the cost of the rotor cover 23, which creates another problem that the strength of the rotor cover 23 is decreased against the centrifugal force. Further, another step is required to magnetize the permanent magnet 22, which further increases the processing cost. Yet further, upon scraping the motor, recycability is low, since the permanent magnet is used.